Ethylene oxide process

ABSTRACT

When supported silver materials are being prepared, deposits of from about 4.0 × 10 -   5  to about 8.0 × 10 -   3  gram equivalent weights per kilogram of finished product of ionic, higher alkali metals, i.e., ionic potassium, rubidium or cesium simultaneously with the deposit of silver improves the selectivity of these materials as catalysts for ethylene oxide production. Superior results are obtained with rubidium or cesium.

This application is a continuation-in-part of copending application Ser.No. 471,398, filed May 20, 1974 abandoned, which is acontinuation-in-part of copending application Ser. No. 317,349 filedDec. 21, 1972, now abandoned, which is a continuation-in-part ofapplication Ser. No. 216,188, filed Jan. 7, 1972, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improved silver catalysts for the productionof ethylene oxide, their preparation, and their use in ethylene oxideprocesses.

2. The Prior Art

Materials consisting of silver upon supports are known to be usefulcatalysts for the production of ethylene oxide by the controlledincomplete oxidation of ethylene with molecular oxygen. A great varietyof modifications have been proposed to improve the activity andselectivity of these catalysts. These modifications have involved, forexample, the supports employed, the method of production, the physicalform of the silver on the support and the addition of additives to thecatalyst.

The alkali metals and their salts have been repeatedly proposed asadditives for various silver ethylene oxide catalysts. Carter, in U.S.Pat. No. 2,125,333, issued Aug. 2, 1938, was among the first to disclosealkali metal addition. He specified the use of "small amounts " ofalkali metals, including both sodium or potassium, in his silvercatalyst. Later patents elaborated on this disclosure, but often withcontradictory teachings. McNamee et al in U.S. Pat. No. 2,238,474,issued Apr. 15, 1941, disclosed that while 100 ppm by weight to 24% byweight of sodium improved silver catalysts, these amounts of potassiumhad a detrimental effect in catalyst performance. Sears, Jr et al inU.S. Pat. No. 2,615,900, issued Oct. 28, 1952, cites a large number ofpromoters useful in broad weight ranges, but makes no distinction in theeffectiveness of the various promoters. Saken, in U.S. Pat. No.2,671,764, issued Mar. 9, 1954 disclosed the use of large amounts ofalkali metal sulfates. Saken in U.S. Pat. No. 2,765,283, issued Oct. 2,1956, proposed adding from 1 ppm by weight to 2000 ppm by weight of aninorganic chlorine compound to the catalyst support prior to theaddition of silver to improve the finished catalyst. Saken's inorganicchlorine compounds included the alkali metal salts of chlorine acids,especially sodium chloride. Carlson et al in U.S. Pat. No. 2,773,844,issued Dec. 11, 1956 discloses a multistep silver deposition process butis not concerned with simultaneous deposition of alkali metals. Gould etal, in U.S. Pat. No. 2,799,687, issued July 16, 1957, disclosed thatwhen from about 20 ppm by weight to about 1.6% by weight of inorganichalide (sodium chloride or preferably potassium chloride) are added asseparate solid particles to a fluidized bed of supported silvercatalyst, the halide acts as a suppresant, inhibiting the catalystactivity. Hosoda et al in U.S. Pat. No. 3,144,416, issued Aug. 11, 1964,cites a number of promoter materials, but gives no limits on theirconcentration. Kriger et al in U.S. Pat. No. 3,563,913 issued Feb. 16,1971, generally discloses the use of alkali and alkaline earth metals aspromoters, listing specifically lithium with no reference to cesium,rubidium or potassium. He notes that these promoters are preferablyadded to the catalyst support before the latter is impregnated with thesilver compound containing solution. Long in U.S. Pat. No. 3,575,888,issued Apr. 20, 1971, disclosed the use of aluminum oxide supportshaving a pore volume between about 0.15 and 0.30 ml/gm and surface areabelow about 10 m² /gm. Nielson in U.S. Pat. No. 3,702,259, issued Nov.7, 1972 uses certain organic amine solubilizing/reducing agents toproduce uniformly spaced, adherent, hemispherical deposits of metallicsilver on catalyst supports.

The prior art clearly recognizes that alkali metal compound additionchanges, for better or worse, the character of a silver ethylene oxidecatalyst. There is no recognition of real advantages for any specialamounts of alkali metal addition, 1 ppm beng considered the equivalentof 2,000 ppm, for example. Moreover, there is no recognition of anyspecial chemical advantages for the addition of certain alkali metalsover other alkali metals. Further, there is no recognition that thesimultaneous deposition of the alkali metal with the silver gives anyspecial advantage over a sequential deposition scheme.

STATEMENT OF THE INVENTION

It has now been found that certain materials consisting essentially ofsilver deposited on a refractory support exhibit improved selectivity ascatalysts for the partial oxidation of ethylene to ethylene oxide. Thesematerials are formed when from about 4.0 × 10⁻ ⁵ to about 8.0 × 10⁻ ³gram equivalent weights per kilogram (based on the entire catalystweight) of ions of rubidium or cesium, is deposited on the catalystsupport coincidentally with the deposit of silver. It has been foundthat when potassium is similarly employed, it permits production of acatalyst of improved selectivity for the above mentioned reaction, butthat the highest level of selectivity obtainable with potassium modifiedcatalysts at otherwise comparable conditions typically is lower thanthat obtainable with rubidium or cesium-modified catalysts. In contrast,it has been found that ions of the lower alkali metals, lithium andsodium, fail to cause commercially significant improvement at theseadditions.

DETAILED DESCRIPTION OF THE INVENTION

The Catalyst of this Invention

When supported silver catalyst materials are prepared by thesimultaneous deposition of rubidium or cesium with the silver on thesupport material, a catalyst superior in selectivity for ethylene oxideproduction is produced.

Catalysts in accord with this invention comprise a porous refractorysupport having deposited on its exterior and interior (pore) surfacesfrom about 2% to about 20% by weight, based on total catalyst, of silverand certain amounts of ionic rubidium or cesium or mixtures thereof.Catalysts prepared with cesium are slightly superior to those preparedwith rubidium as the alkali metal. Generally inferior, but still veryuseful, improvements are obtained with potassium as alkali metal, asclaimed in copending application serial numbers (K-2028M and K-2028Q).Mixtures of alkali metals may be employed and the use of minorproportion of potassium relative to rubidium and/or cesium is within thecompass of the present invention. For purposes of the presentspecification, the effective metals potassium, rubidum and cesium arereferred to as "higher alkali metals" and rubidium and cesium, themetals claimed herein, as "higher alkali metals of this invention".

The higher alkali metals of this invention are present on the catalystsin the form of their cations, rather than as the extremely active freealkali metals. Silver, on the other hand, is present on the finishedcatalysts as silver metal. The exact form of the alkali metal on thefinished catalyst is not known. However, the heat treatment given to theimpregnated support in order to reduce the silver salts to metallicsilver will most likely convert the alkali metal salts to an oxide oroxidic compound. The exact form of this oxide compound has not beendetermined, but most likely it will depend upon the alkali metalspresent and the type of support used. By way of explanation and withoutintending to limit the scope of the invention, it is hypothesized thatthe oxide compound somehow forms a complex with the catalyst supportsurface rather than simply being deposited inertly upon the supportsurface. For example, when the alkali metal is deposited upon an aluminasupport it is thought that an aluminum-alkali metal-oxygen complex isformed. For sake of clarity in this specification and claims the amountof alkali metal oxide compound present on the catalyst support has beenexpressed as the gram equivalent weight of the alkali metal rather thanthe oxide. While the catalysts of this invention will have deposited onthe support surface oxides or compounds of rubidium, cesium or mixturesthereof, minor amounts or proportions of oxides or compounds potassiumrelative to rubidium and/or cesium is within the compass of this presentinvention. The alkali metal present in final form on the support in theform of an oxide or compound will be selected from the group consistingof cesium, rubidium, mixtures or rubidium and cesium, cesium combinedwith a minor amount of potassium, rubidium combined with a minor amountof potassium or mixtures of rubidium and cesium combined with a minoramount of potassium said minor amounts being relative to said rubidumand/or cesium. Alternatively stated, the alkali metal is present infinal form on the support in the form of an oxide or oxide compound inwhich the said oxide or compound consists of oxides or compounds ofcesium or rubidium or mixtures thereof optionally combined with a minorproportion of the oxide or compound of potassium relative to saidrubidium and cesium.

The amount of the higher alkali metal (or metals) present on thecatalyst surface is critical. For any of the higher alkali metals ofthis invention, it appears that amounts of less than about 4.0 × 10⁻ ⁵gram equivalent weights (gew) per kilogram or greater than about 8.0 ×10⁻ ³ gew per kilogram provide no significant improvement. The amount ofhigher alkali metals of this invention present on the catalystpreferably ranges from about 2.0 × 10⁻ ⁴ to about 6.5 × 10⁻ ³ gew perkilogram of finished catalyst. As in more specifically detailed below,the amount of higher alkali metals required for this invention is afunction of the surface area of the catalyst. Thus, the amount of higheralkali metals of this invention desirably present will fall at thehigher limit of the desired range for the higher surface area supportsand at the lower limit for the lower surface area supports of thisinvention below about 0.8 m² /g. A desired range of alkali metals ofthis invention is from about 0.3 × 10⁻ ³ to about 3.2 × 10⁻ ³ gew perkilogram of finished catalyst and more preferrably between about 0.35 ×10⁻ ³ to about 3 × 10⁻ ³ gew per kilogram of finished catalyst and evenmore preferably from about 5.0 × 10⁻ ⁴ to about 2.4 × 10⁻ ³ gew perkilogram of finished catalyst. The original application Ser. No.216,188, filed Jan. 7, 1972, now abandoned, disclosed a range of higheralkali metals of from about 0.5 × 10⁻ ³ to about 2.4 × 10⁻ ³ gew perkilogram of finished catalyst and preferably from about 0.75 × 10⁻ ³ toabout 2.3 × 10⁻ ³ gew per kilogram of finished catalyst. Otherundetermined experimental factors or other unknown variables may alsocontribute to minor variations in the concentration ranges of each ofthe higher alkali metals of this invention wherein optimum selectivityis obtained when the catalysts are employed in partial oxidation ofethylene to thylene oxide.

It must be made clear that the amounts of rubidium or cesiumsimultaneously deposited are not necessarily the total amounts of thesemetals present in the catalysts. They are the amounts of these alkalimetals which are present on the surface of the catalyst and which areintentionally added to the catalysts coincidentally with the addition ofsilver. It is not unusual for substantial amounts, often up to about10,000 ppm wt. of an alkali metal (usually potassium) to be presentwithin the porous support due to use of support materials containingnaturally occuring alkali metals or inadvertent alkali metal additionduring support manufacture. These amounts of alkali metal present in thesupport in non-leachable form, rather than on the surface coincidentallyadded with the silver, do not appear to contribute to the improvedperformance of catalysts in accord with this invention and are neglectedin determination of alkali metal concentrations.

Catalysts according to this invention preferably contain from about 2 toabout 20% by weight based on the total catalyst of silver as silvermetal. Preferably they contain from about 3 to about 15 percent and mostpreferably from about 4 to about 13% by weight of silver. The use oflarger amounts of silver is not excluded but is generally economicallyunattractive. The silver is deposited over the interior and exteriorsurfaces of the catalyst support and should be evenly dispersed overthese surfaces.

The exact physical form of the silver upon the support can vary and doesnot appear to be critical to the invention. Very excellent results areobtained with the controlled surface alkali metal content catalyst ofthis invention, however, when the silver is present in the form ofuniformly space, discontinuous, adherent, substantially hemispherical,discrete particles having a uniform diameter of less than one micron(10,000A). Best results are obtained with this type of catalyst when thesilver particles have diameters of from about 1,000 to about 10,000A andmost preferred catalysts have silver particles of an average diameter inthe range of from about 1500 to about 7500A.

The support employed in these catalysts in its broadest aspects isselected from the large number of conventional, porous, refractorycatalyst carriers or support materials which are essentially inert inthe presence of the ethylene oxidation feeds, products and reactionconditions. Such conventional materials may be of natural or syntheticorigin and preferably are of a macroporous structure, that is, astructure having a surface area below about 10 m² /g and preferablybelow about 2 m² /g. These support materials typically have an apparentporosity of greater than 20%. Very suitable supports comprise those ofsiliceous and/or aluminous composition. Specific examples of suitablesupports are the aluminum oxides (including the materials sold under thetrade name "Alundum"), charcoal, pumice, magnesia, zirconia, kieselguhr,fuller's earth, silicon carbide, porous agglomerates comprising siliconand/or silicon carbide, silica, magnesia, selected clays, artificial andnatural zeolites and ceramics. Refractory supports particularly usefulin preparation of catalysts in accordance with this invention comprisethe aluminous materials, in particular those containing alpha alumina.In the case of alpha alumina-containing supports, preference is given tothose having a specific surface area as measured by the B.E.T. method offrom about 0.03 m² /g to about 2.0 m² /g and an apparent porosity asmeasured by conventional mercury or water absorption techniques of fromabout 25% to about 50% by volume. The B.E.T. method for determiningspecific surface area is described in detail in Brunauer, S., Emmet,P.H., and Teller, E., J. Am. Chem. Soc., 60, 309-16 (1938).

When certain types of alpha alumina-containing supports are employed,the advantages of the special alkali metal addition of this inventionare especially emphasized. These alpha alumina supports have relativelyuniform pore diameters and are more fully characterized by having (1)B.E.T. specific surface areas of from about 0.1 m² /g to about 2.0 m²/g, preferably about 0.1 m² /g to about 0.8 m² /g and (2) apparentporosities of from about 42% to about 56%, preferably from about 46% toabout 52%. Specific examples of these most advantageous supports are thesupports marketed by Norton Company as "Alundum" grades LA-956, LA-556and LA-4118. Further details on these supports are given in Table I.

                  TABLE I                                                         ______________________________________                                        Support           LA-956   LA-5556  LA-4118                                   Surface Area, m.sup.2 /g                                                                        .17      .24      .35                                       Specific Pore Volume, cc/g                                                                      .19      .25      .31                                       Median Pore Diameter, Microns                                                                   2.5      4.4      5.7                                       % of Pores between 1.5 and 15                                                 Microns           79       81       47                                        ______________________________________                                    

When these types of alumina are used, it has been found that the amountof higher alkali metal simultaneously deposited with the silver neededfor optimum selectivity to ethylene oxide is directly related to thesurface area of the support. For example, if the ratio of the cesiumvalue at maximum selectivity to B.E.T. surface area is calculated, avalue of about ##EQU1## is obtained. While the scientific explanationfor the above mentioned relationship between the support surface areaand the selectivity is not understood, it is expected that similarrelationships will hold for any of the other large number ofconventional porous catalyst supports used to prepare the catalysts ofthis invention.

Regardless of the character of the support used, it is preferably shapedinto particles, chunks, pieces, pellets, rings, spheres, and the like ofa size suitable for employment in fixed bed applications. Conventionalcommercial fixed bed ethylene oxidation reactors are typically in theform of a plurality of parallel elongated tubes (in a suitable shell)approximately 1 to 2 inches in diameter and 24 to 45 feet long filledwith catalyst. In such reactors, it is desirable to employ a supportformed into a rounded shape, such as for example, spheres, pellets,rings, tablets, and the like, having diameters of from about 0.1 inch toabout 0.8 inch.

THE CATALYST PREPARATION

The catalysts of this invention are prepared by a technique which causesthe coincidental deposit of silver and the desired alkali metal upon thecatalyst support surfaces.

A great variety of methods for adding silver to supports are known. In atypical method, the support may be impregnated with an aqueous solutionof silver nitrate dried and the silver reduced with hydrogen orhydrazine as described in U.S. Pat. No. 3,575,888, issued Apr. 20, 1971,to Long. In another technique the support may be impregnated with anammoniacal solution of silver oxalate or carbonate and the silver metalformed by thermally decomposing the salt. Silver may be added as well bythe technique disclosed in U.S. Pat. No. 3,702,259 of Nielsen, whereinthe support is impregnated with special aqueous solutions of silversalts and combinations of ammonia, vicinal alkanolamines and vicinalalkyldiamines and then thermally treated. Other possible methods foradding silver include impregnating a support with anethanolamine-containing solution of silver salt and then reducing, asdisclosed by Japanese Pat. No. 19606/1971, or by adding a slurry of fineparticles of silver carbonate to the support and thermally decomposingas described by Endler in U.S. Pat. NO. 3,043,854 issued July, 10, 1962.In each of these techniques, silver is added to the support when thesupport is contacted with a liquid phase, either a silver solution or aslurry of silver particles. With these techniques, an excellent methodfor adding the desired alkali metals which enables their simultaneousdeposit with silver is to dissolve them as suitable salts in this liquidphase in an amount regulated to give the required alkali metal additionto the finished catalyst when the support is contacted therewith.Suitable higher alkali metal salts generally include all those which aresoluble in the silver-depositing liquid phase. In this regard, nounusual effectiveness is observed with use of any particular anion inthe alkali metal salts. For example, nitrates, nitrites, chlorides,iodides, bromates, bicarbonates, oxalates, acetates, tartrates,lactates, isopropoxides, and similar common alkali metal salts may beused. However, alkali metal salts should be avoided which react with thesilver present in the liquid phase such as to cause silver salts toprecipitate prematurely out of an impregnating solution. For example,cesium chloride should not be used in impregnation techniques which usean aqueous silver nitrate solution but can be used in such a techniquewith an aqueous solution of silver amine complexes from which silverchloride will not precipitate.

In general terms, catalysts in accord with this invention are preparedby (A) contacting a suitable solid porous refractory support with aliquid phase which contains an amount of silver, either as silvercompounds dissolved in the liquid phase or as a slurry of silvercompound particles, in an amount sufficient to deposit from 2 to 20% byweight of silver on the support surface and an amount of dissolved saltsof cesium, rubidium or mixtures thereof optionally combined with a minoramount of the salts of potassium relative to said rubidium and cesiumsufficient to deposit from 4.0 × 10.sup.⁻⁵ to 8.0 × 10.sup.⁻³ gew perkilogram of these higher alkali metals as salts on the surface of thesupport, thereby coincidentally depositing these amounts of silvercompounds and alkali metal salts upon the catalyst surface; and (B)thermally treating the resulting silver compound- and higher alkalimetal salt-containing product in the presence of a reducing agent toconvert the silver compound to silver metal. Suitable impregnatingsolutions contain, for example, from about 3 to about 40% by weight ofsilver salts and from about 25 to about 500 ppm by weight of alkalimetal. The exact concentrations employed generally may require someroutine experimentation since the amount of alkali metal deposited froma solution, which is critical, will depend in part on the porosity ofthe catalyst support. However, methods of varying the amount of silverand/or alkali metal deposited are conventional, as is the analyticaldetermination of the amount of the materials actually deposited.

Also within the scope of the invention is an alternative method ofcoincidentally depositing the silver and higher alkali metal salt on thesupport surface which provides a ready means of controlling the amountof higher alkali metal deposited within the limits of the invention,i.e., 4.0 × 10.sup.⁻⁵ to 8.0 × 10.sup.⁻³ gew per kilogram of finishedcatalyst. This method involves deposition of larger than requiredamounts of the higher alkali metal, that is, in excess of 8.0 ×10.sup.⁻³ gew per kilogram of finished catalyst coincidentally with thesilver according to the general procedure described above followed bycontacting the catalyst particles so prepared with an anhydrous alkanolof 1 to 2 carbon atoms. The higher alkali metals contemplated by thisinvention are soluble in the alkanol solvents described to a sufficientdegree that one or more washings with the alkanol solvents willselectively remove the excess coincidentally deposited higher alkalimetal such that the amount remaining intact on the support surface fallswithin the concentration range critical to the invention. This methodthen provides a ready means of adjusting the higher alkali metalconcentration from levels in excess of those described by the invention,whether the result of purposeful or inadvertent actions, to specificconcentrations within the limits of the invention, by a process which isreadily applicable to large plant scale operations.

The alkali metal addition of this invention is especially effective whenused in conjunction with silver catalyst preparation techniques whereinsilver is added to the support from a basic solution, particularly froma nitrogenous base-containing basic solution. Examples of thesenitrogenous bases are ammonia, the alkylamines and the alkanolamines.

Thus, in a preferred embodiment, catalysts of this invention areprepared by (A) adding to a porous aluminous support from about 3% byweight to about 15% by weight of silver in the form of water-solublesilver salts and from about 1.0 × 10.sup.⁻⁴ gew per kilogram to about6.5 × 10.sup.⁻³ gew per kilogram of higher alkali metal of thisinvention (rubidium and/or cesium) in the form of water-soluble salts bycontacting the support with an alkaline aqueous solution of the silverand alkali metal salts; and (B) maintaining the product of step (A) at atemperature of from about 100° C to about 500° C in the presence of areducing agent for a period sufficient to convert the silver salts tosilver metal.

In a particularly preferred modification, the alkali metal addition ofthis invention is used in conjunction with catalyst preparationtechniques such as those disclosed in U.S. Pat. No. 3,702,259 ofNielsen. This preferred preparation method involves impregnation of analumina support with certain aqueous alkali metal silver salt solutionsand a subsequent thermal reduction of the silver salts. The impregnationsolution consists essentially of:

A. a silver salt of a carboxylic acid,

B. an organic amine alkaline solubilizing/reducing agent,

C. a salt of rubidium or cesium, and

D. an aqueous solvent.

Suitable carboxylic acid silver salts include silver carbonate and thesilver salts of mono- and polybasic carboxylic and hydroxycarboxylicacids of up to about 16 carbon atoms. Silver carbonate and silveroxalate are particularly useful silver salts, with silver oxalate beingmost preferred.

An organic amine solubilizing/reducing agent is present in theimpregnating solution used in this preparation method. Suitable organicamine silver-solubilizing/reducing agents include lower alkylenediaminesof from 1 to 5 carbon atoms, mixtures of a lower alkanolamine of from 1to 5 carbon atoms with a lower alkylenediamine of from 1 to 5 carbonatoms, as well as mixtures of ammonia with lower alkanolamines or loweralkylene-diamines of from 1 to 5 carbons. Four groups of organic aminesolubilizing/reducing agents are preferred. They are the following:

A. vicinal alkylenediamines of from 2 to 4 carbon atoms;

B. mixtures of (1) vicinal alkanolamines of from 2 to 4 carbon atoms and(2) vicinal alkylenediamines of from 2 to 4 carbon atoms;

C. mixtures of vicinal alkylenediamines of from 2 to 4 carbon atoms andammonia; and

D. mixtures of vicinal alkanolamines of from 2 to 4 carbon atoms andammonia.

These preferred solubilizing/reducing agents are generally added in theamount of from 0.1 to 10 moles per mole of silver present.

Very preferred as solubilizing/reducing agents are:

A. ethylenediamine,

B. ethylenediamine in combination with ethanolamine,

C. ethylenediamine in combination with ammonia, and

D. ethanolamine in combination with ammonia.

ethylenediamine in combination with ethanolamine is most preferred.

When ethylenediamine is used as the sole solubilizing/reducing agent, itis necessary to add amounts of the amine in the range of from 0.1 to 5.0moles of ethylenediamine per mole of silver.

When ethylenediamine and ethanolamine together are used assolubilizing/reducing agent, it is suitable to employ from 0.1 to 3.0moles of ethylenediamine per mole of silver and from 0.1 to 2.0 moles ofethanolamine per mole of silver.

When ethylenediamine or ethanolamine is used with ammonia, it isgenerally useful to add at least about two moles of ammonia per mole ofsilver and very suitable to add from about 2 to about 10 moles ofammonia per mole of silver. The amount of ethylenediamine orethanolamine employed then is suitably from 0.1 to 2.0 moles per mole ofsilver.

Suitable higher alkali metal salts are those of inorganic and organiccarboxylic acids. It is often convenient to employ as the higher alkalimetal salt the carboxylic acid salt corresponding to the silvercarboxylate used, as for example, to use rubidium or cesium oxalate whensilver oxalate is the silver source.

As already noted, it is essential that only certain controlled amountsof the higher alkali metals of this invention be present. These amountsare achieved by either controlled addition of alkali metal to an alkalimetal-free silver solution, by controlled removal of alkali metal froman alkali metal-rich solution or by controlled removal of alkali metalfrom the support surface after deposition of larger than requiredamounts of alkali metal. For example, a cesium-containing silver oxalatesolution can be prepared by two methods. Silver oxide can be reactedwith mixture of ethylenediamine and oxalic acid to yield a solution ofsilver oxalate ethylenediamine complex to which is then optionally addedother amines, such as ethanolamine and controlled amounts of potassium.Alternatively, silver oxalate can be precipitated from cesium oxalateand silver nitrate and then repeatedly rinsed to remove adhered cesiumsalts until the desired cesium content is reached. The cesium-containingsilver oxalate is then solubilized with ammonia and/or amines. When thesupport is contacted with these solutions, silver (as salts) and higheralkali metals (as salts) deposit simultaneously upon the supportsurfaces.

Subsequent to the use of any of these methods, the impregnated supportis then heated at a temperature of from 100° to 375°, preferably from125° to 325° C, for the time, typically 1/2 to 8 hours, required todecompose the silver salt and form the adherent particulate deposit ofmetallic silver on the surfaces. Lower temperatures do not adequatelydecompose the silver salt and should be avoided. More than onetemperature may be employed.

In one special embodiment a silver oxalate-ethylenediamine complexsolution containing larger than required amounts of alkali metal can beemployed. In this case the concentration of alkali metal is reduced,after decomposition on the support and subsequent thermal treatment toconvert the silver compound to metallic silver by contacting thesupported catalyst with a lower alkanol, i.e., methanol or ethanol,according to the general procedure described above.

ETHYLENE OXIDE PRODUCTION

The higher alkali metal-promoted silver catalysts have been shown to beparticularly selective catalysts in the direct oxidation of ethylenewith molecular oxygen to ethylene oxide. The conditions for carrying outsuch an oxidation reaction in the presence of the silver catalysts ofthe present invention broadly comprise those described in the prior art.This applies, for example, to suitable temperatures, pressures,residence times, diluent materials such as nitrogen, carbon dioxide,steam, argon, methane or other saturated hydrocarbons, the presence orabsence of moderating agents to control the catalytic action, forexample, 1,2-dichloroethane, vinyl chloride or chlorinated polyphenylcompounds, the desirability of employing recycle operations or applyingsuccessive conversion in different reactors to increase the yields ofethylene oxide, and any other special conditions which may be selectedin processes for preparing ethylene oxide. Pressures in the range offrom about atmospheric to about 500 psi are generally employed. Higherpressures may, however, be employed within the scope of the invention.Molecular oxygen employed as reactant is obtained from conventionalsources. The suitable oxygen charge may consist essentially ofrelatively pure oxygen, a concentrated oxygen stream comprising oxygenin major amount with lesser amounts of one or more diluents such asnitrogen, argon, etc., or another oxygen-containing stream such as air.The use of the present novel silver catalysts in ethylene oxidationreactions is in no way limited to the use of specific conditions amongthose which are known to be effective.

In a preferred application of the silver catalysts of the inventionethylene oxide is produced when an oxygen-containing gas of not lessthan 95% oxygen is contacted with ethylene in the presence of thepresent catalysts at a temperature in the range of from 210° C to 285° Cand preferably 225° C to 270° C.

The resulting ethylene oxide is separated and recovered from thereaction products by conventional methods known and used in the art. Useof the silver catalysts of the invention in ethylene oxide productionprocesses gives higher overall ethylene oxidation selectivities toethylene oxide at a given ethylene conversion than are possible withconventional catalysts.

While the reason for these higher selectivities observed with catalystsof this invention is not fully understood, experiments have indicatedthat conventional silver catalysts (not containing higher alkali metals)cause ethylene oxide to combust after formation while silver catalystscontaining from about 4.0 × 10.sup.⁻⁵ to about 8.0 × 10.sup.⁻³ gew perkilogram of codeposited higher alkali metal do not cause as extensiveethylene oxide combustion.

Preparation of catalysts according to the invention and their use in theproduction of ethylene oxide will be further described by the followingexamples which are provided for illustration and are not to be construedas limiting the invention.

EXAMPLE I AND COMPARATIVE EXPERIMENT A

A. A potassium-containing catalyst as claimed in copending applicationsSer. No. 621,268, filed Oct. 10, 1975 (Catalyst A) was prepared. Assupport for this catalyst was employed 5/16-inch diameter rings ofalpha-alumina. This support contained 99.3% by weight alpha-alumina,0.4% silica, and 0.3% of other metal oxides and had a surface area of0.24 m² /g and an apparent porosity of 48-49% by volume. This supporthad a median pore diameter of 4.4 microns as determined by mercuryporosimetry. Eight percent of its pores had diameters in the range offrom 1.5 to 15 microns. The tradename of this support was NortonCompany's Alundum, type LA-5556.

The support was impregnated with an aqueous solution of silver saltcontaining a controlled amount of potassium. Reagent grade silver oxidewas mixed with an aqueous solution of reagent grade oxalic aciddissolved in ethylenediamine to form an about 2 molar solution of Ag₂(EN)₂ C₂ O₄. Ten percent by volume of ethanolamine (about 0.4 moles ofethanolamine per mole of silver) was then added to complete thesolubilizing/reducing agent combination. This solution contained about22% by weight silver. Potassium nitrate, to achieve 190 ppm weight ofpotassium, was added to the solution. The catalyst support wasimpregnated with this potassium-containing silver solution, vacuum beingapplied to ensure complete saturation. Excess liquid was drained off andthe support was immediately placed in a forced air oven at 290° C to drythe catalyst and reduce the silver salt to silver metal. Total heatingtime was about 3 hours. The silver content of the catalyst wasdetermined to be 7.8% by weight and the potassium present on thecatalyst surface was found to be 60 ppm (0.0015 gew of potassium perkilogram of catalyst).

The form of the silver deposits on the catalyst was examined by electronmicroscope and found to be discrete particles having a uniform diameterof from 0.2 to 0.4 microns (2000 to 4,000A). These particles wereuniformly spaced on the interior and exterior of the support. Repeateddropping and shaking of catalyst A showed that the silver particles weretightly adherent to the support surface.

B. For purposes of comparison the preparation of part A was repeatedwith the difference that no alkali metal salt was added to theimpregnating solution. Analysis verified that no more than 5 ppm ofpotassium was present in the solution. This catalyst (catalyst A')contained 7.8% by weight of silver.

C. The potassium-modified catalyst (catalyst A) and the alkalimetal-free catalyst not in accordance with the invention (catalyst A')were comparatively tested as catalysts for production of ethylene oxide.In a representative experiment, 5/16-inch rings of catalyst A werecrushed and 3.5 grams of 30/40 mesh particles of crushed catalyst werecharged to a 0.20-inch diameter by 5-inch long reaction tube. A mixtureof air and ethylene was passed over the catalyst in the presence of achlorine-containing moderator at the following reaction conditions:

    ______________________________________                                        Pressure, psig          200                                                   Space Velocity, hours.sup.-.sup.1                                                                    3300                                                   Ethylene in charge, % m                                                                              30                                                     Ethylene/O.sub.2 ratio 3.75                                                   Moderator Concentration,                                                      ppm of equivalent Chlorine                                                                           10-15                                                  ______________________________________                                    

The reaction temperature was adjusted to provide an oxygen conversion of52% and the selectivity to ethylene oxide was determined. With catalystA, a temperature of 253° C was required to reach a standard oxygenconversion (52%). Selectivity to ethylene oxide was 78%. By comparison,the catalyst not in accordance with the invention (catalyst A') achieveda selectivity of only 69%.

EXAMPLE II

A second potassium-containing, catalyst (catalyst B) was prepared usinga method similar to the method of Example I, part A. The support ofExample I was used.

This support was impregnated as in Example I with an aqueous solution ofpotassium and silver salts, which in this Example was prepared by thefollowing different technique. An aqueous solution of reagent gradesilver nitrate was mixed with stirring with an aqueous solution ofreagent grade potassium oxalate. Silver oxalate precipitate wascollected and washed repeatedly with deionized water until a potassiumcontent of 8 ppm potassium per percent weight silver was noted. Thispotassium-containing silver oxalate was then dissolved in aqueousethylenediamine and used to impregnate the support as in Example I. Thefinished catalyst contained 7.8% by weight of silver and 62 ppm byweight of coincidentally-deposited potassium. When catalyst B wasemployed as an ethylene oxide catalyst in accord with Example I, it gavestandard conversion at 253° C and 78.6% selectivity.

EXAMPLE III

A. Using the general catalyst preparation technique and feedstocks ofExample I, a group of silver catalysts were prepared having varyingpotassium contents. The compositions of these catalysts are listed inTable II.

Each of these catalysts was subjected to a prolonged test in a pilotscale ethylene oxide reactor 13/4 inches in diameter and 35 feet long.

The conditions of these experiments were as follows:

    ______________________________________                                        Pressure, psig         210                                                    Temperature, ° C                                                                              245-260                                                Space velocity, hours.sup.-.sup.1                                                                    3300                                                   Ethylene in feed, %    30                                                     Ethylene/O.sub.2 ratio 3.5                                                    Oxygen conversion, %   52                                                     Optimum moderator                                                             concentration ppm of                                                          equivalent chlorine    11-14                                                  ______________________________________                                    

The results of these experiments are given in Table II.

                                      TABLE II                                    __________________________________________________________________________                   Added Potassium                                                               Content                                                                       gew of                                                                        potassium                                                                            ppm   Oxidation                                              Silver Content                                                                          per kilogram                                                                         wt.   selectivity to                                    Catalyst                                                                           % wt.     of catalyst                                                                          potassium                                                                           ethylene oxide, %                                 __________________________________________________________________________    3-A  7.8 ± 0.3                                                                             0.00026                                                                             10    68.5                                              3-B  "          0.00037                                                                             14    69.4                                              3-C  "          0.00062                                                                             24    70.6                                              3-D  "          0.00065                                                                             27    70.4                                              3-E  "         0.0011 42    74.3                                              3-F  "         0.0012 44    75.0                                              3-G  "         0.0016 63    76.3                                              3-H  "         0.0016 64    76.2                                              3-I  "         0.0018 72    76.0                                              3-J  "         0.0021 82    73.3                                              __________________________________________________________________________

EXAMPLE IV

Catalysts according to this invention, containing varying amounts ofrubidium as the higher alkali metal component, were prepared using thefeedstocks and general preparation technique of Example I. Instead ofadding potassium to the impregnating solution, rubidium as rubidiumnitrate, was added. The catalyst compositions, so prepared, were testedas ethylene oxide catalysts using the apparatus and techniques ofExample I, part C. The compositions of these catalysts along withresults of the ethylene oxide preparations utilizing these catalysts aregiven in Table III below:

                                      TABLE III                                   __________________________________________________________________________                  Added Rubidium                                                                          Reactor tempera-                                                    Content   ture to achieve                                                                        Oxidation                                         Silver Content                                                                         gew per                                                                            ppm wt.                                                                            52% O.sub.2                                                                            selectivity to                               Catalyst                                                                           %wt.     kilogram                                                                           rubidium                                                                           Conversion ° C                                                                  ethylene oxide                               __________________________________________________________________________    4-A  7.8       0.00068                                                                            58  254.0    73.4                                         4-B  7.8      0.0011                                                                              90  257.5    77.5                                         4-C  7.8      0.0015                                                                             125  249.0    79.7                                         4-D  7.8      0.0018                                                                             155  258.5    79.0                                         4-E  7.8      0.0018                                                                             155  255.5    79.5                                         4-F  7.8      0.0021                                                                             179  263.0    79.9                                         4-G  7.8      0.0021                                                                             180  260.0    78.3                                         4-H  7.8      0.0021                                                                             209  267.5    76.4                                         4-I  7.8      0.0029                                                                             247  286.0    74.9                                         4-J  7.8      0.0032                                                                             273  315.0    67.4                                         __________________________________________________________________________

EXAMPLE V

Catalysts containing varying amounts of cesium as the higher alkalimetal component were prepared using the feedstocks and generalpreparation technique of Example I. Instead of adding potassium to theimpregnating solution, cesium as cesium nitrate, was added. The catalystcompositions, so prepared, were tested as ethylene oxide catalysts usingthe apparatus and techniques of Example I, part C. The compositions ofthese catalysts along with results of the ethylene oxide preparationsutilizing these catalysts are given in Table IV below:

                                      TABLE IV                                    __________________________________________________________________________                  Added Cesium                                                                            Reactor tempera-                                                    Content   ture to achieve                                                                        Oxidation                                         Silver Content                                                                         gew per                                                                            ppm wt.                                                                            52% O.sub.2                                                                            selectivity to                               catalyst                                                                           % wt.    kilogram                                                                           cesium                                                                             Conversion ° C                                                                  ethylene oxide                               __________________________________________________________________________    5-A  7.8      0.00042                                                                            55   248.5    77.2                                         5-B  7.8      0.00061                                                                            81   256.2    79.4                                         5-C  7.8      0.00066                                                                            88   256.0    80.0                                         5-D  7.8      0.00068                                                                            90   262.5    79.3                                         5-E  7.8      0.00071                                                                            95   251.0    79.8                                         5-F  7.8      0.00071                                                                            94   252.0    79.8                                         5-G  7.8      0.00074                                                                            96   256.0    80.0                                         5-H  7.8      0.00074                                                                            97   257.0    80.3                                         5-I  7.8      0.00079                                                                            105  252.0    81.0                                         5-J  7.8      0.00079                                                                            107  257.5    80.2                                         5-K  7.8      0.00079                                                                            112  254.5    79.6                                         5-L  7.8      0.00081                                                                            108  247.0    79.5                                         5-M  7.8      0.0012                                                                             157  261.5    78.2                                         5-N  7.8      0.0016                                                                             208  273.5    76.2                                         5-O  7.8      0.0020                                                                             265  325.0    65.2                                         __________________________________________________________________________

Comparative Experiments D and E

Two series of catalysts were prepared using the feedstocks and generalpreparation method of Example I with the exception that in one seriesvarying amounts of sodium were employed as dopant and in the otherseries varying amounts of lithium were used.

These catalysts all contained about 7.8% by weight silver and from 0.001to 0.002 gew per kilogram of catalyst of lithium or sodium.

These catalysts were tested as ethylene oxidation catalysts using theequipment and technique of Example I, part C. Within the limits ofexperimental error, no improvement in selectivity was noted with thesecatalysts.

EXAMPLE VI

Catalysts containing varying amounts of rubidium as the higher alkalimetal component were prepared using the feedstocks and generalpreparation technique of Example I. Instead of adding potassium to theimpregnating solution, rubidium as rubidium nitrate was added. Thecatalyst compositions, so prepared, were tested as ethylene oxidecatalysts using the apparatus and techniques of Example I, part C. Thecompositions of these catalysts along with results of the ethylene oxidepreparations utilizing these catalysts are given in Table V below:

                                      TABLE V                                     __________________________________________________________________________                  Added Rubidium                                                                          Reactor tempera-                                                    Content   ture to achieve                                                                        Oxidation                                         Silver Content                                                                         gew per                                                                            ppm wt.                                                                            52% O.sub.2                                                                            Selectivity to                               Catalyst                                                                           %wt.     kilogram                                                                           rubidium                                                                           Conversion ° C                                                                  ethylene oxide                               __________________________________________________________________________    6-A  10.4     0.00015                                                                            13   254.5    72.5                                         6-B  10.4     0.00048                                                                            41   252.0    76.0                                         6-C  10.4     0.00064                                                                            55   255.5    78.2                                         6-D  10.4     0.00094                                                                            80   263.5    77.8                                         6-E  10.4     0.0012                                                                             103  249.0    80.1                                         6-F  10.4     0.0016                                                                             134  263.0    79.2                                         __________________________________________________________________________

EXAMPLE VII

Catalysts containing varying amounts of cesium as the higher alkalimetal component were prepared using the feedstocks and generalpreparation technique of Example I. The catalyst support used here had ahigher surface area than that of Example I. This support contained 99.6%by weight of alpha-alumina, <.01% silica and < 0.4% other metal oxidesand had a surface area of 1.07 m² /g as measured by the B.E.T. method.The name of this support was LA-4102, manufactured by Norton Company.

Instead of adding potassium to the impregnating solution, cesium ascesium nitrate, was added. The catalyst compositions, so prepared, weretested as ethylene oxide catalysts using the apparatus and techniques ofExample I, part C. The compositions of these catalysts along withresults of the ethylene oxide preparations utilizing these catalysts aregiven in Table VI below:

                                      TABLE VI                                    __________________________________________________________________________                  Added Cesium                                                                  Content at Maximum                                                            Selectivity Reactor tempera-                                                                           Oxidation                                   silver   gew per                                                                              ppm wt.                                                                            ture to achieve                                                                            Selectivity to                         Catalyst                                                                           content % w                                                                            kilogram                                                                             cesium                                                                             52% O.sub.2 Conversion ° C                                                          Ethylene Oxide                         __________________________________________________________________________    7-A  8.01     0      0    248.5        69.8                                   7-B  8.01     .00245 325  250.0        75.0                                   7-C  8.01     .00311 413  260.0        75.9                                   7-D  8.01     .00417 554  259.0        77.4                                   7-E  8.01     .00494 656  287.5        77.0                                   7-F  8.01     .00543 722  321.5        68.0                                   __________________________________________________________________________

EXAMPLE VIII

Catalysts with differing silver contents and containing varying amountsof potassium as the higher alkali metal component were prepared usingthe feedstocks and general preparation technique of Example I. Thesupport was the same as that used in Example I, part A. The catalystcompositions, so prepared, were tested as ethylene oxide catalysts usingthe apparatus and techniques of Example 1, part C. The compositions ofthese catalysts along with results of the ethylene oxide preparationsutilizing these catalysts are summarized in Table VII below for thealkali metal concentrations giving the maximum selectivity at a givensilver loading. The maximum selectivity for each silver loading wasdetermined by, for example, a procedure such as that of Example VII.

                                      TABLE VII                                   __________________________________________________________________________                Added Potassium                                                               Content at Maximum                                                            Selectivity Reactor tempera-                                                                           Oxidation                                     Silver gew per                                                                             ppm wt                                                                              ture to achieve                                                                            Selectivity                              Catalyst                                                                           Content % w                                                                          kilogram                                                                            potassium                                                                           52% O.sub.2 Conversion ° C                                                          Ethylene Oxide                           __________________________________________________________________________    8-A  4.1    0.00109                                                                             43    262.2        78.8                                     8-B  7.8    0.00166                                                                             65    253.5        79.0                                     8-C  10.4   0.00266                                                                             104   256.0        79.2                                     __________________________________________________________________________

EXAMPLE IX

Catalysts with different silver loadings and containing varying amountsof cesium as the higher alkali metal component were prepared using thefeedstocks and general preparation technique of Example I. The supportwas the same as that used in Example I, part A. Instead of addingpotassium to the impregnating solution, cesium as cesium nitrate, wasadded. The catalyst compositions, so prepared, were tested as ethyleneoxide catalysts using the apparatus and techniques of Example I, part C.The compositions of these catalysts along with results of the ethyleneoxide preparations utilizing these catalysts are summarized in TableVIII below for the alkali metal concentration giving the maximumselectivity of a given silver loading. The maximum selectivity for eachsilver loading was determined by, for example, a procedure such as thatof Example VII.

                                      TABLE VIII                                  __________________________________________________________________________                Added Cesium                                                                  Content at Maximum                                                            Selectivity                                                                             Reactor tempera-                                                                          Oxidation                                        Silver gew per                                                                            ppm wt.                                                                           ture to achieve                                                                            Selectivity to                              Catalyst                                                                           Content % w                                                                          kilogram                                                                           cesium                                                                            52% O.sub.2 Conversion ° C                                                          Ethylene Oxide                              __________________________________________________________________________    9-A  11.60  .00111                                                                             147 248.5        81.3                                        9-B  10.40  .00109                                                                             145 268.0        80.0                                        9-C  7.80   .00081                                                                             107 257.5        80.2                                        9-D  5.38   .00096                                                                             128 272.0        79.1                                        9-E  4.00   .00123                                                                             163 266.5        79.0                                        9-F  1.56   .00047                                                                              62 319.0        75.3                                        __________________________________________________________________________

EXAMPLE X

Catalysts using alumina supports with varying surface areas andcontaining cesium as the higher alkali metal component were preparedusing the feedstocks and general preparation technique of Example I.Instead of adding potassium to the impregnating solution, cesiumnitrate, was added. The catalyst compositions, so prepared, were testedas ethylene oxide catalysts using the apparatus and techniques ofExample I, part C. The compositions of these catalysts along withresults of the ethylene oxide preparations utilizing these catalysts aresummarized in Table IX below for the alkali metal concentration givingthe maximum selectivity at a given surface area. This maximumselectivity was determined by, for example, a procedure such as that ofExample VII.

                                      TABLE IX                                    __________________________________________________________________________    Support            Added Cesium                                                                             Reactor Tem-                                    BET            Silver                                                                            Content at Maximum                                                                       perature to                                                                            Oxidation                              Surface   Median                                                                             Con-                                                                              Selectivity                                                                              achieve 52%                                                                            Selectivity                                 Area Pore tent                                                                              gew per                                                                            ppm of                                                                              O.sub.2 Conversion                                                                     to Ethylene                            Catalyst                                                                           m.sup.2 /g                                                                         Diameter                                                                           % wt                                                                              kilogram                                                                           cesium                                                                              20 C     oxide %                                __________________________________________________________________________    10-A 0.20 3.90 10.4                                                                              .00109                                                                             145   268.0    80.0                                   10-B 0.31 1.58 10.5                                                                              .00180                                                                             238   269.5    81.2                                   10-C 0.42 1.60 9.1 .00299                                                                             397   272.0    80.2                                   10-D 1.07 0.97 8.01                                                                              .00417                                                                             554   259.0    77.4                                   10-E 1.31 --   5.4 .00670                                                                             890   257.5    76.2                                   10-F 4.50 0.28 8.5 .01357                                                                             1803  297.5    64.8                                   10-G 6.67 --   10.5                                                                              .04229                                                                             5621  277.0    42.4                                   __________________________________________________________________________

EXAMPLE XI

A catalyst in accord with this invention (Catalyst 11-A) was preparedusing a non-alumina support. The support was in the form of 3/16 inchdiameter pellets. This support contained 93% by weight silica and 75% byweight alumina and had a surface area of 0.6 m² /g, a median median porediameter of 7 microns as determined by mercury porosimetry, and anapparent porosity of 51% by volume. 80% of its pores had diameters inthe range of from 3 to 17 microns. The support was impregnated accordingto the technique of Example 1, part A. The weight percent of potassiumwas 68 ppm (.0017 gew/kilogram).

A catalyst not in accord with this invention (11-B) was prepared similarto Catalyst 11-A, above, with the exception that no alkali metal waspresent in the finished catalyst.

When Catalyst 11-A was employed as an ethylene oxidation catalyst inaccord with Example I, it gave standard conversion at 256.0° C and 75.0%selectivity. Catalyst 11-B under the same test conditions gave standardconversion at 257.5° C and 65.9% selectivity.

We claim as our invention:
 1. In the process for the production ofethylene oxide wherein ethylene is contacted in vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at anelevated temperature of from 210° to about 285° C in the presence of afixed bed of a silver metal-containing catalyst, the improvement whichcomprises employing as said catalyst an article comprising a porousrefractory support having deposited on its exterior and pore surfacesfrom about 2% by weight to about 20% by weight of silver and,coincidentally deposited with silver, a total of from about 4.0 × 10⁻ ⁵gew per kilogram of total catalyst to about 8.0 × 10⁻ ³ gew per kilogramof total catalyst of alkali metal present in final form on the supportin the form of an oxide in which said oxide consists of oxides of cesiumor rubidium or mixtures thereof.
 2. The process in accord with claim 1wherein the among of alkali metal coincidentally deposited with thesilver is in the range from about 2.0 × 10⁻ ⁴ to about 6.5 × 10⁻ ³ gewper kilogram of total catalyst.
 3. The process in accord with claim 1wherein the amount of alkali metal coincidentally deposited with thesilver is in the range from about 3.0 × 10⁻ ⁴ to about 3.2 × 10⁻ ³ gewper kilogram of total catalyst.
 4. The process in accord with claim 1wherein the amount of alkali metal coincidentally deposited with thesilver is in the range from about 5.0 × 10⁻ ⁴ to about 2.4 × 10⁻ ³ gewper kilogram of total catalyst.
 5. The process in accord with claim 12wherein the alkali metal coincidentally deposited with the silver isrubidium.
 6. The process in accord with claim 2 wherein the alkali metalcoincidentally deposited with the silver is cesium.
 7. The process inaccord with claim 4 wherein the alkali metal coincidentally depositedwith the silver is rubidium.
 8. The process in accord with claim 4wherein the alkali metal coincidentally deposited with the silver iscesium.
 9. The process in accord with claim 2 wherein the porousrefractory support is alpha alumina and the alkali metal coincidentallydeposited with the silver is rubidium.
 10. The process in accord withclaim 2 wherein the porous refractory support is alpha alumina and thealkali metal coincidentally deposited with the silver is cesium.
 11. Inthe process for the production of ethylene oxide wherein ethylene iscontacted in vapor phase with an oxygen-containing gas at ethylene oxideforming conditions at an elevated temperature of from 210° to about 285°C in the presence of a fixed bed of a silver metal-containing catalyst,the improvement which comprises employing as said catalyst an articleconsisting essentially of a porous alpha alumina support having asurface area of from about 0.03 m² /g to about 2.0 m² /g and havingdeposited on its exterior and pore surfaces from about 2% by weight toabout 20% by weight of silver and, coincidentally deposited with thesilver, a total of from about 2.0 × 10⁻ ⁴ gew per kilogram of totalcatalyst to about 6.5 × 10⁻ ³ gew per kilogram of total catalyst ofalkali metal present in final form on the support in the form of anoxide in which the said oxide consists of oxides of cesium or rubidiumor mixtures thereof.
 12. The process in accord with claim 11 wherein thesurface area of the alpha alumina support is from about 0.1 m² /g toabout 2.0 m² /g.
 13. The process in accord with claim 12 wherein thesilver is present in an amount of from about 3% by weight to about 15%by weight.
 14. The process in accord with claim 12 wherein the alkalimetal coincidentally deposited with the silver is rubidium.
 15. Theprocess in accord with claim 12 wherein the alkali metal coincidentallydeposited with the silver is cesium.
 16. The process in accord withclaim 14 wherein the concentration of said rubidium is from about 5.0 ×10⁻ ⁴ to about 2.4 × 10⁻ ³ gew per kilogram of total catalyst.
 17. Theprocess in accord with claim 15 wherein the concentration of said cesiumis from about 5.0 × 10⁻ ⁴ to about 2.4 × 10⁻ ³ gew per kilogram of totalcatalyst.
 18. In the process for the production of ethylene oxidewherein ethylene is contacted in vapor phase with an oxygen-containinggas at ethylene oxide forming conditions at an elevated temperature offrom 210° to about 285° in the presence of a fixed bed of a silvermetal-containing catalyst, the improvement which comprises employing assaid catalyst an article comprising a porous support of siliceouscomposition having a surface area of from about 0.03 m² /g to about 2.0m² /g and having deposited on its exterior and pore surfaces from about2% by weight to about 20% by weight of silver and, coincidentallydeposited with the silver, a total of from about 2.0 × 10⁻ ⁴ gew perkilogram of total catalyst to about 6.5 × 10⁻ ³ gew per kilogram oftotal catalyst of alkali metal present in final form on the support inthe form of an oxide in which the said oxide consists of oxides ofcesium or rubidium or mixtures thereof.
 19. The process in accord withclaim 18 wherein the amount of alkali metal coincidentally depositedwith the silver is in the range from about 5.0 × 10⁻ ⁴ to about 2.4 ×20⁻ ³ gew per kilogram of total catalyst.
 20. In the process for theproduction of ethylene oxide wherein ethylene is contacted in vaporphase with an oxygen-containing gas at ethylene oxide forming conditionsat an elevated temperature of from 210° to about 285° in the presence ofa fixed bed of silver metal-contaning catalyst, the improvement whichcomprises employing as said catalyst an article prepared by a processwhich comprises impregnating a porous refractory catalyst support with asolution comprising solvent, silver salt sufficient to deposit on saidsupport from 2% by weight to 20% by weight of silver and salts of one ormore higher alkali metals sufficient to deposit on said support a totalof from about 4.0 × 10⁻ ⁵ gew per kilogram of total catalyst to about8.0 × 10⁻ ³ gew per kilogram of total catalyst of at least one alkalimetal in the form of a salt in which the said salt consists of salts ofcesium or rubidium or mixtures thereof; separating the impregnatedsupport from excess impregnating solution and thereafter reducing thesilver salt to silver metal at a temperature of from about 100° to about500° C.
 21. The process in accord with claim 20 wherein the amount ofalkali metal added to the catalyst support is in the range from about5.0 × 10⁻ ⁴ to about 2.4 × 20⁻ ³ gew per kilogram of total catalyst. 22.The process in accord with claim 21 wherein the alkali metalcoincidentally deposited with the silver is rubidium.
 23. The process inaccord with claim 21 wherein the alkali metal coincidentally depositedwith the silver is cesium.
 24. In the process for the production ofethylene oxide wherein ethylene is contacted in vapor phase with anoxygen-containing gas at ethylene oxide forming conditions at anelevated temperature of from 210° to about 285° C in the presence of afixed bed of a silver metal-containing catalyst, the improvement whichcomprises employing as said catalyst an article prepared by a processwhich comprisesa. impregnating a porous refractory catalyst support witha solution comprising solvent, silver salt sufficient to deposit from 2by weight to 20% by weight of silver on said support and salts of one ormore higher alkali metals in which the said salts consist of salts ofcesium or rubidium or mixtures thereof sufficient to deposit an amountof said alkali metal in excess of 8.0 × 10⁻ ³ gew per kilogram of totalcatalyst on said support; b. separating the impregnated support; c.reducing the silver salt present on said impregnated support to silvermetal at a temperature of from about 100° to about 500° C; d. contactingsaid impregnated support wherein silver is present as silver metal oneor more times with an alkanol of 1 or 2 carbon atoms to selectivelyremove an amount of higher alkali metal present on said impregnatedsupport such that the impregnated support so contacted contains 4.0 ×10⁻ ⁵ to 8.0 × 10⁻ ³ gew of higher alkali metal per kilogram of catalystpresent in final form on the support in the form of an oxide in whichthe said oxide consists of oxides of cesium or rubidium or mixturesthereof; and e. separating the impregnated support from the alkanolcontacting solution and drying to substantially remove the residualalkanol present in and on the impregnated support.
 25. In the processfor the production of ethylene oxide wherein ethylene is contacted invapor phase with a oxygen-containing gas an ethylene oxide formingconditions at an elevated temperature of from 210° to about 285° C inthe presence of a fixed bed of a silver metal-containing catalyst, theimprovement which comprises employing as said catalyst an articleprepared by a process which comprisesa. simultaneously adding to aporous refractory catalyst support from about 3% by weight to about 15%by weight of silver in the form of silver salts and from about 2.0 × 10⁻⁴ gew per kilogram of total catalyst to about 6.5 × 10⁻ ³ gew perkilogram of total catalyst of higher alkali metals in the form ofwater-soluble salts in which the said salts consist of salts of cesiumor rubidium or mixtures thereof, by contacting said support with analkaline aqueous solution of said silver and higher alkali metal salts;and b. maintaining the product of step (a) at a temperature of fromabout 100° C to about 500° C in the presence of a reducing agent,thereby converting the silver salts to silver metal.
 26. The process inaccord with claim 25 wherein said alkaline aqueous solution contains anorganic amine.
 27. The process in accord with claim 26 wherein saidsilver salts are silver salts of carboxylic acids.
 28. The process inaccord with claim 25 wherein the porous refractory catalyst supportconsists essentially of alpha alumina.
 29. The process in accord withclaim 25 wherein the porous refractory catalyst support is a siliceoussolid.